1. Field of the Invention
The present invention relates to a catalytic system usable for the copolymerization of at least one conjugated diene and at least one monoolefin, to a process for preparing this catalytic system, to a process for preparing a copolymer of at least one conjugated diene and at least one monoolefin involving the use of said catalytic system, and to such a copolymer. The invention applies in particular to the copolymerization of a conjugated diene with an alpha-olefin and/or ethylene.
2. Description of Related Art
It has long been difficult to carry out the copolymerization of a conjugated diene and a monoolefin, such as an alpha-olefin (i.e. comprising by definition at least three carbon atoms, unlike ethylene which is not an alpha-olefin), due to the different coordination indices exhibited by conjugated dienes and monoolefins with regard to Ziegler Natta type catalytic systems based on transition metals.
It has been known since the 1970s to prepare alternating copolymers of a conjugated diene, such as butadiene or isoprene, and an alpha-olefin, such as propylene, by means of such catalytic systems based on vanadium or titanium. Reference will be made, for example, to the article “Furukawa, J. in Alternating Copolymers, Cowie, J. M. G., ed.; Plenum Press: New York, 1985; pp. 153-187” which mentions the use of a catalytic system based on a derivative of vanadium and an aluminum compound for obtaining these copolymers.
One major disadvantage of these catalytic systems is that they must be prepared at very low temperatures (approximately −70° C.) and that they entail the use of a likewise low copolymerization temperature, being between −30° C. and −50° C. Using higher temperatures for the copolymerization results in deactivation of these catalytic systems and in a reduction in the molecular masses of the copolymers obtained.
In order to obtain alternating copolymers of butadiene and propylene exhibiting higher molecular masses with improved activity and greater control of the degree of alternation, German patent specification DE-A-270 6118 teaches the use of catalytic system comprising a vanadium dialkoxyhalide and a trialkylaluminum.
One major disadvantage of these catalytic systems based on vanadium is again that copolymerization must be carried out at low temperature.
Non-alternating conjugated diene/alpha-olefin copolymers, such as butadiene/propylene or butadiene/ethylene/propylene copolymers, have also been produced in the past, the copolymerization reactions being performed at temperatures higher than ambient temperature. To this end, use has been made either of homogeneous catalytic systems based on a halogenated trialkylaluminum and a vanadium derivative (see German patent specifications DE-A-253 4496 and DE-A-20O 1367 which relate to obtaining butadiene/ethylene/propylene terpolymers with a reduced content of trans-1,4 butadiene units) or of a derivative of titanium and phosgene (see the article “Furukawa, J. et al., J. Polym. Sci., Polym. Chem. Ed. 1973, 11, p. 629” which relates to obtaining random copolymers), or of catalytic systems based on TiCl4 supported on a magnesium halide (see European patent specification EP-A-171 025).
It will be noted that these latter supported catalytic systems have the twin drawback of giving rise, on the one hand, to the formation of a gel in the resultant copolymers and, on the other hand, to reduced molar contents of inserted butadiene, typically of less than 15%.
The attempt has also been made to prepare butadiene/ethylene/propylene terpolymers by means of homogeneous catalytic systems comprising conventional metallocenes of group IV of the periodic table which satisfy the formula Cp2MX2 (see international patent specification WO-A-88/04672 and the article “Galimberti et al., Makromol. Chem. 1991, 192, p. 2591”).
One major disadvantage of these catalytic systems of the formula Cp2MX2 is that the butadiene considerably limits activity and is inserted in only very small quantities. It will be noted that the copolymers obtained in this manner comprise cyclic units (cyclopentane units).
European patent specification EP-A-891 993 proposes a catalytic system for the copolymerization of a monoolefin having 2 to 12 carbon atoms and at least one conjugated diene monomer, which system comprises constituent (a) below and at least one compound selected from among constituents (b), (c) and (d) below:
(a) a transition metal complex satisfying any one of the following formulae:(Cp1-Z-Y)MX1X2 or alternatively (Cp1Cp2-Z)MX1X2, where
M is any one of the following metals: Ti, Zr, Hf, Rn, Nd, Sm, Ru,
Cp1 and Cp2 are each a cyclopentadienyl, indenyl or fluorenyl group,
Y is a ligand containing an atom of oxygen, nitrogen, phosphorus or sulfur,
Z represents C, O, B, S, Ge, Si, Sn or a group containing any one of these atoms,
X1 and X2 each represent an anionic or neutral ligand which is a Lewis base;
(b) a compound which reacts with the metal M of (a) to form an ionic complex;
(c) an organoaluminum compound; and
(d) an aluminoxane.
It will be noted that the copolymers obtained in this document EP-A-891 993 comprise a low molar content of inserted conjugated diene (less than 10%) and that they always comprise cyclic units (of the cyclopentane and cyclopropane type).
Catalytic systems specifically comprising a lanthanide complex and enabling the copolymerization of conjugated dienes and alpha-olefins have also been reported in the literature. Kaulbach et al. have described the copolymerization of butadiene/octene or dodecene with a neodymium octoate complex in Angew. Makromol. Chem. 1995, 226, p. 101. Visseaux M. et al have described the copolymerization of alpha-olefin/conjugated diene (butadiene or isoprene) with a lanthanide allyl complex in Macromol. Chem. Phys. 2001, 202, p. 2485.
One major disadvantage of these latter catalytic systems is that the molar content of inserted alpha-olefin is always low, being less than 20%.
It is furthermore known to copolymerise ethylene and a conjugated diene, such as butadiene, by means of halogenated lanthanide complexes which can be alkylated in situ in the polymerization medium via a co-catalyst. Accordingly, European patent specification EP-A-1 092 731 in the name of the present applicants teaches the use to this end of a catalytic system comprising:                on the one hand, an organometallic complex represented by one of the following formulae:        

where Ln represents a lanthanide metal having an atomic number which may range from 57 to 71,
where X represents a halogen which may be chlorine, fluorine, bromine or iodine,
where, in the formula A′, two ligand molecules Cp1 and Cp2, each consisting of a substituted or unsubstituted cyclopentadienyl or fluorenyl group, are attached to the metal Ln,
where, in the formula B′, a ligand molecule consisting of two substituted or unsubstituted cyclopentadienyl or fluorenyl groups Cp1 and Cp2 linked to one another by a bridge P of the formula MR2, where M is an element from column IVa of Mendeleev's periodic table, and where R is an alkyl group comprising from 1 to 20 carbon atoms, is attached to the metal Ln and                on the other hand, a co-catalyst selected from the group comprising an alkylmagnesium, an alkyllithium, an alkylaluminum, a Grignard reagent and a mixture of these constituents.        